linux-kernel - Re: [PATCH] crypto: x86/twofish-3way

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Message-ID: <20171219075443.tdpt2l72eelhpi7j@gmail.com>
Date:   Tue, 19 Dec 2017 08:54:43 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jussi Kivilinna <jussi.kivilinna@....fi>, x86@...nel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Eric Biggers <ebiggers@...gle.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage


* Eric Biggers <ebiggers3@...il.com> wrote:

> There may be a small overhead caused by replacing 'xchg REG, REG' with
> the needed sequence 'mov MEM, REG; mov REG, MEM; mov REG, REG' once per
> round.  But, counterintuitively, when I tested "ctr-twofish-3way" on a
> Haswell processor, the new version was actually about 2% faster.
> (Perhaps 'xchg' is not as well optimized as plain moves.)

XCHG has implicit LOCK semantics on all x86 CPUs, so that's not a surprising 
result I think.

Thanks,

	Ingo